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                  <p class="p_Heading1"><span class="f_Heading1">Sequence Elements</span></p>



  
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                <p class="p_BodyText"><span class="f_BodyText">A <a href="sequencediagram.htm">Sequence diagram</a> models a dynamic view of the interactions between model elements at runtime.</span></p>
<p class="p_BodyText"><span class="f_BodyText">Sequence diagrams are commonly used as explanatory models for Use Case scenarios. By creating a Sequence diagram with an Actor and elements involved in the Use Case, you can model the sequence of steps the user and the system undertake to complete the required tasks. An element in a Sequence diagram is usually either an Actor (the stimulus that starts the interaction) or a collaborating element.</span></p>
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<td valign="bottom" width="955" bgcolor="#efeff7" style="width:955px; background-color:#efeff7;"><p class="p_TiporNote" style="border-top: none; border-right: none; border-left: none;"><span class="f_TiporNote">Note:</span></p>
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<td width="955" bgcolor="#f7f7ff" style="width:955px; background-color:#f7f7ff;"><p class="p_TiporNoteText"><span class="f_TiporNoteText">A Sequence diagram is often attached directly under the Use Case to which it refers. This helps keep elements together, both in the model and when documentation is produced. To do this, right-click the Use Case on the diagram and select the </span><span class="f_MenuKeyField">Advanced | Make Composite</span><span class="f_TiporNoteText"> menu option.</span></p>
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<p class="p_BodyText"><span class="f_BodyText">The example below shows some possible elements of Sequence diagrams and their stereotyped display.</span></p>
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<li><span class="f_Bulletlist1" style="font-style: italic;">Actor</span><span class="f_Bulletlist1"> - An instance of an actor at runtime.</span></li>
<li><span class="f_Bulletlist1" style="font-style: italic;">Lifeline</span><span class="f_Bulletlist1"> - An Object element with the stereotype Lifeline.</span></li>
<li><span class="f_Bulletlist1" style="font-style: italic;">Boundary</span><span class="f_Bulletlist1"> -</span><span class="f_Bulletlist1"> Represents a user interface screen or input/output device.</span></li>
<li><span class="f_Bulletlist1" style="font-style: italic;">Entity -</span><span class="f_Bulletlist1"> A persistent element - typically implemented as a database table or element.</span></li>
<li><span class="f_Bulletlist1" style="font-style: italic;">Control</span><span class="f_Bulletlist1"> -</span><span class="f_Bulletlist1"> The active component that controls what work gets done, when and how.</span></li>
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<p class="p_TextIndentedL2"><img src="sequence%20diagram%20elements.png" width="352" height="235" border="0" alt="Sequence Diagram Elements"></p>
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<td valign="bottom" width="955" bgcolor="#efeff7" style="width:955px; background-color:#efeff7;"><p class="p_TiporNote" style="border-top: none; border-right: none; border-left: none;"><span class="f_TiporNote">Tip:</span></p>
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<td width="955" bgcolor="#f7f7ff" style="width:955px; background-color:#f7f7ff;"><p class="p_TiporNoteText"><span class="f_TiporNoteText">Use Sequence diagrams early in analysis to capture the flow of information and responsibility throughout the system. Messages between elements eventually become method calls in the Class model.</span></p>
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